Process of producing a V2 O5 -and-alkali-metal-sulfate-containing catalyst for oxidizing SO2 to SO3

ABSTRACT

A catalyst is produced in two stages in that prefabricated carrier bodies are impregnated with solutions which contain vanadium and alkalies. When the impregnated carrier bodies have been dried, the conversion activity of the catalyst is increased further in that the catalyst is activated under oxidizing conditions at a temperature of 700° to 1000° C.

FIELD OF THE INVENTION

My present invention relates to a process for producing a vanadium oxideand alkali metal catalyst for the oxidation of SO₂ to SO₃ in whichcarrier bodies are impregnated with solutions that contain vanadium andalkalies and the carrier bodies are subsequently dried at elevatedtemperatures.

BACKGROUND OF THE INVENTION

The impregnation of prefabricated carrier bodies affords variousadvantages over the production of a catalyst in a single process stage,in which the carrier material and the active materials are mixed and aresubsequently shaped to catalyst bodies.

The prefabricated carrier bodies usually consist of SiO₂ and can beproduced without restriction as to the active substances. As a result,carrier bodies that are uniform in composition and have a large internalsurface area and a high mechanical strength can be made. Theimpregnation results in a uniform distribution of the active substancesthroughout the carrier bodies so that the resulting catalyst bodies areuniform in composition and rejection of poor quality products isavoided. The catalysts have high conversion activities.

The production of catalysts by an impregnation of prefabricated carrierbodies has been described in commonly assigned U.S. Pat. No. 4,431,573of Feb. 14, 1984 and corresponding German patent document - openapplication DE-OS No. 30 33 319. The carrier bodies in this system areimpregnated with an impregnating solution which contains 600 to 1100grams H₂ SO₄ and 140 to 830 grams V₂ O₅ and alkali metal sulfate perliter. The impregnation is effected at a temperature between 60° C. andthe boiling temperature of the impregnating solution. The impregnatedcarrier bodies are dried with agitation at a temperature of up to 700°C. The catalysts have high conversion activities even at lowtemperatures and are mechanically stable even at high temperatures.

German Application No. 30 22 894 describes such a process for theproduction of catalysts for use in a fluidized bed. In that process thecarrier bodies do not contain Al₂ O₃ in excess of 4% and are impregnatedwith a solution which has a low sulfuric acid content and containsvanadyl oxysulfate and potassium hydrogen sulfate. In this solutionvanadium is present in tetravalent form. The impregnation is followed bydrying at a temperature of 150° to 250° C. The carrier bodies or theimpregnated carrier bodies are hardened at a temperature which is not inexcess of 600° C. and preferably between 200° and 400° C.

Catalysts may also be produced in a two-stage process comprising animpregnation with alkaline or neutral solutions (German PatentPublication No. 12 81 407, German patent document - open applicationDE-OS No. 15 42 177, Austrian Pat. No. 116,361, U.S. Pat. No. 1,941,426;B. Waeser "Die Schwefelsaurefabrikation", 1961, Verlag Friedrich Viewegund Sohn, Brunswick, Germany, pages 252 to 258). This process, however,involves an attack of the alkaline solution on the SiO₂ content of thecarrier bodies. If neutral solutions are employed, the poor solubilityof the active substances requires an impregnation in a plurality ofstages so that it is difficult to adjust a uniform content of activesubstances.

OBJECT OF THE INVENTION

It is the object of my invention further to increase the conversionactivity of catalysts made in two stages by an impregnation ofprefabricated carrier bodies.

DESCRIPTION OF THE INVENTION

This object is attained in accordance with the invention by activatingthe dried catalyst under oxidizing conditions at a temperature between700° and 1000° C. The temperature between 700° and 1000° C. must bepresent within the catalyst. The optimum temperature is a temperature atwhich all active substances are molten and which does not exceed thehighest permissible temperature of the carrier bodies, i.e. thetemperature at which their internal surface area is altered-or thesurface change threshold. The highest permissible temperature may dependon the time of action.

If the temperature rises at a high rate, a higher temperature will bepermissible than in the case of a slower temperature rise.

The highest permissible temperature may also depend on the compositionof the impregnating solution if the chemical attack of the impregnatingsolution on the carrier bodies increases in intensity as the temperaturerises. The optimum temperature for a given catalyst is determined by adetermination of the conversion activity.

Before alkali-impregnated carrier bodies are activated, the activesubstances must be acidified. Such acidification is not required afteran impregnation with an acid solution or with sulfatic active substancesin a neutral solution.

The term "oxidizing conditions" means that the gas atmosphere must havean oxidizing activity on the active substances, particularly as thecatalyst is cooled.

The activation can be effected as a separate step when the impregnatedcatalyst has been dried. The drying and activating steps may be carriedout in succession in a common piece of equipment, e.g. in a rotary kilnor on a belt after a preliminary drying effected with agitation.

Alternatively, the impregnating, drying and activating steps may beperformed in a single piece of equipment, such as a rotary kiln. Heatingmay be effected with flue gases in direct contact with the catalyst ifsaid gases have an oxidizing activity. Heating may also be effected byan indirect heat exchange. In the latter case the resulting SO₂-containing atmosphere must be exhausted to provide oxidizing conditionsby an ingress of air, for example.

In accordance with a preferred further feature the activation iseffected at a temperature between 750° and 950° C. A particularlyeffective activation will be achieved by a treatment in that temperaturerange.

In accordance with a further preferred feature the activation iseffected with countercurrent flows of the catalyst and the gasatmosphere and/or with an exhausting of the gas atmosphere. The gasatmosphere is introduced into the activating unit from its discharge endor is correspondingly exhausted. That practice will result in aparticularly effective activation because the catalyst will be exposedto a reliably oxidizing gas atmosphere as the temperature rises anddrops.

In accordance with yet a further preferred feature the catalyst is heldat the predetermined activating temperature for 5 to 120 minutes. Theresidence time at this temperature will depend on the highesttemperature. The residence time will be longer at a lower temperatureand will be shorter at a higher temperature. This practice will resultin a particularly effective activation.

In accordance with a still further preferred feature the process isapplied to the activation of a dried catalyst produced by animpregnation of carrier bodies with acid solutions which contain alkalimetal sulfate, vanadium sulfate and free sulfuric acid. This practicewill result in a particularly effective activation because the SiO₂content of the carrier bodies will not be attacked by the impregnatingsulfuric acid solutions. The process disclosed in U.S. Pat. No.4,431,573 and German Application No. 30 33 319 is particularlyeffective.

SPECIFIC EXAMPLES

The invention will be explained more fully with reference to thefollowing Examples.

EXAMPLE 1

In accordance with German Application No. 30 33 319, a catalyst wasproduced on a laboratory scale as follows:

A commercially available SiO₂ carrier (spherical bodies 5.3 mm indiameter) was impregnated with an impregnating solution containing

152.5 g/l V₂ O₅

310.2 g/l K₂ SO₄

113.7 g/l Cs₂ SO₄

900 g/l H₂ SO₄

The impregnating solution was used in excess and was decanted afterhaving acted for 20 minutes. The carriers had a liquid absorptioncapacity of 720 ml per kg of carrier.

The carrier bodies were then partially dried with agitation in atumbling evaporator at 150° C. in a vacuum produced by water jets andwere finally dried in a second step in a drying cabinet at 300° C. andnormal pressure.

Part of the catalyst was aftertreated in that it was placed at roomtemperature in a thin layer into a bowl, which was then introduced intoa muffle furnace, which was at room temperature, and was subsequentlyheated to 750° C. in 75 minutes. By comparison measurements it was foundthat the difference between the measured gas temperature and thetemperature in the interior of the catalyst is relatively small andamounts to 15° to 20° C. during the temperature rise and reaches valuesbelow 5° C. after a holding time of about 5 minutes.

After a holding at 750° C. for half an hour, the muffle furnace wasopened and the bowl with the catalyst was taken and permitted to cool inthe air.

Activity measurements had the following results:

    ______________________________________                                                         SO.sub.2 conversion                                                           %         at °C.                                      ______________________________________                                        Catalyst dried at 300° C.                                                                 98.82       415                                            Catalyst aftertreated at 750° C.                                                          99.06       405                                            ______________________________________                                    

This shows that the activity was distinctly and significantly improvedby the aftertreatment.

EXAMPLE 2

The process of Example 1 was repeated but the impregnating solution wasused in a measured rather than an excessive amount.

The carrier bodies agitated in the tumbling evaporator at 110° C. weresupplied with the measured amount of the impregnating solution in such amanner that the solution was distributed as uniformly as possible. Inorder to ensure that the impregnating solution was completely absorbedby the carriers, the amount of the impregnating solution was restrictedto 90% of the maximum liquid absorption capacity of the carriers. Thecontents of the active substances in the impregnating solution werecorrespondingly increased so that the finished catalyst had the samecontents of active substances as the catalyst produced in Example 1.

650 ml of an impregnating solution having the following composition wereused per kg of the carriers:

168.9 g/l V₂ O₅

343.6 g/l K₂ SO₄

125.9 g/l Cs₂ SO₄

900 g/l H₂ SO₄

The carrier bodies were dried in a vacuum in the same tumblingevaporator immediately after the impregnation. The further treatment wasthe same as in Example 1 and the results were also the same as thoseobtained in Example 1.

EXAMPLE 3

The process of Example 2 was repeated. The drive of the tumblingevaporator was interrupted during the impregnation so that part of theimpregnating solution was supplied to the carrier bodies when they wereat rest. This resulted in a highly irregular distribution, which wasapparent from different colors of the carrier bodies. The dryingresulted in a mixture of catalyst parts having greatly varying contentsof active substances.

The further treatment was the same as in Examples 1 and 2. The followingresults were obtained:

    ______________________________________                                                         SO.sub.2 conversion                                                           %         at °C.                                      ______________________________________                                        Catalyst dried at 300° C.                                                                 98.41       420                                            Catalyst aftertreated at 750° C.                                                          99.06       405                                            ______________________________________                                    

As was to be expected, the disturbance of the impregnating step had theresult that the merely dried catalyst had a lower conversion activitythan the catalyst obtained by the optimum treatment in Examples 1 and 2.That difference in quality was entirely compensated by theaftertreatment, which resulted in the same final quality as was achievedin Examples 1 and 2. The carrier bodies were uniform in color.

This shows that the use of the aftertreatment permits the entireproduction of the catalyst to be simplified because the results of aprocessing which is not perfectly optimum on the quality of the catalystwill be compensated by the aftertreatment. As a result, the economy ofthe catalyst production is increased because charges which must berejected will be virtually avoided.

EXAMPLE 4

The unsatisfactory charge obtained in Example 3 was used for a detailedinvestigation of the influence of the conditions of the aftertreatment.As has been described in Example 1, the aftertreatment comprised in eachcase:

Placing the sample into the muffle furnace at room temperature

Raising the temperature to the desired treating temperature

Holding at the treating temperature

Taking the hot sample after the treatment for the desired time andcooling in air.

The following heating-up times were determined in dependence on thetreating temperatures:

    ______________________________________                                        Treating temperature                                                                           Heating-up time                                              °C.       min                                                          ______________________________________                                        600              58                                                           650              63                                                           700              68                                                           750              75                                                           800              79                                                           850              86                                                           900              94                                                           950              102                                                          1000             112                                                          ______________________________________                                    

The influence of the treating conditions is apparent from the followingtable:

    ______________________________________                                        Temp.   Time         SO.sub.2 Conversion                                      °C.                                                                            h            %           at °C.                                ______________________________________                                        --      --           98.41       420                                          600     2            98.49       420                                          600     6            98.68       415                                          600     20           98.82       415                                          600     260          98.86       415                                          650     0.5          98.72       415                                          650     2            98.85       410                                          700     0.5          99.00       410                                          700     2            99.04       410                                          750     0.5          99.06       405                                          750     2            99.06       405                                          800     0.5          99.06       405                                          850     0.5          99.02       410                                          900     0.5          99.00       410                                          950     0.5          98.82       415                                          1000    0.5          98.40       425                                          1000    2            98.12       440                                          ______________________________________                                    

This shows that the activity is progressively increased as thetemperature increases up to an optimum temperature between 750° and 800°C. and that the conversion activity decreases as the temperature isincreased about that optimum value.

The influence of temperature is increased by the time of the treatmentbut this is only of secondary importance.

EXAMPLE 5

The systematic investigation apparent from Example 4 was repeated with adifferent catalyst of lower activity. That catalyst differed from theone used in Example 4 in that it was made with a different carriermaterial. The contents of active substances in percent by weight was thesame. The carrier material which was used was different in severalrespects:

A different SiO₂ raw material was used, which had a B.E.T. surface areathat was only about 1/10 of that of the carrier material used in Example4 (15 m² /g rather than 150 m² /g).

The geometric shape (cylinder) and the dimensions were different so thatthe geometrical surface area was smaller by about 20%.

The production process (shaping, tempering, etc.) was different.

The aftertreatment was performed in some cases in the muffle furnace inthe manner described in Examples 1 and 4, with a holding time of 1/2hour, and in other cases, for comparison, in an indirectly heated rotarykiln.

For this purpose a sample of 1 liter of catalyst was filled into arotary kiln of steel and was heated from room temperature to thetreating temperature and was held at the treating temperature for 1/2hour. After the treating time the hot sample was immediately removedfrom the rotary kiln and was permitted to cool in air. An air stream forcarrying away the evolved gases was passed through the rotary kiln. Inanother test series the air stream was replaced by syntheticallyproduced flue gas (9 vol. % H₂ O, 9 vol. % CO₂, 7 vol. % O₂, 75 vol. %N₂)

The measurement of the activity of the catalysts thus pretreated (theholding temperature was 1/2 hour in all cases) had the followingresults:

    ______________________________________                                                                    Conversion                                        Treating Conditions          Activity                                         Temp.    Equipment Atmosphere   %    at °C.                            ______________________________________                                        --       --        --           95.7 460                                      725      muffle    air          97.8 435                                      725      rotary kiln                                                                             air          98.03                                                                              430                                      725      rotary kiln                                                                             flue gas     98.20                                                                              425                                      775      muffle    air          98.07                                                                              425                                      775      rotary kiln                                                                             air          98.17                                                                              425                                      775      rotary kiln                                                                             flue gas     98.30                                                                              425                                      825      rotary kiln                                                                             air          98.47                                                                              420                                      825      muffle    air          98.33                                                                              425                                      825      rotary kiln                                                                             flue gas     98.59                                                                              420                                      875      muffle    air          98.60                                                                              420                                      875      rotary kiln                                                                             flue gas     98.46                                                                              420                                      925      muffle    air          98.32                                                                              430                                      975      muffle    air          97.9 440                                      1025     muffle    air          96.5 450                                      ______________________________________                                    

The results show:

(a) The increase in activity due to the activating treatment isdifferent for different catalysts. The lower the activity of thecatalyst before the aftertreatment, the larger will be the influence ofthe aftertreatment:

Catalyst of Example 4: From 98.41 to 99.06

Catalyst of Example 5: From 95.74 to 98.60

(b) The optimum temperature of the aftertreatment is different fordifferent catalysts:

Catalyst of Example 4: 750° to 800° C.

Catalyst of Example 5: 825° to 875° C.

(c) Different units of equipment will produce different results. Thevalues obtained with the rotary kiln are generally somewhat higher thanthose obtained with the muffle furnace. This is specific for theequipment.

(d) Indirect or direct heating may be employed. Flue gas has a ratherfavorable influence on the activity of the catalyst.

EXAMPLE 6

The catalyst was produced in batches of 400 liters in a Technikum plant.The carrier material and the impregnating solution were comparable tothose used in Example 2. The carrier material was preheated to 100° to120° C. in a rotary kiln directly heated with a gas burner. During therotation of the rotary kiln the impregnating solution was sprayed ontothe carrier material and the catalyst was subsequently dried by means ofthe gas burner. For this purpose the flue gas temperature was increasedto about 500° C. in about 3 hours, held at that value for 1 hour. Theburner was then shut down and air was sucked through the rotary kiln forabout 2 hours to cool the catalyst until the exhaust gas had atemperature of 100° to 150° C. The catalyst was then removed. Theaftertreatment was carried out in a second rotary kiln, which wasindirectly heated and continuously operated. At a rate of about 30 kg/hthe catalyst was continuously charged by a metering feeder into therotary kiln at a cold end thereof and the catalyst was passed throughthe rotary kiln in dependence on its inclination and on the level towhich it was filled, etc. The catalyst moved through the entiretemperature profile of the rotary kiln in that the catalyst was chargedat a low temperature, heated to a maximum temperature and then cooledsubstantially as it continued its travel in the rotary kiln until itreached the non-heated end of the kiln. The temperature at the hottestportion was regarded as the nominal treating temperature and wasautomatically controlled. The influences of the gas flow pattern and oftemperature were investigated with a view to an optimization of theaftertreatment.

The gas flow pattern was influenced in that a suction was applied to thetwo ends of the rotary kiln in alternation. When the gas was sucked in aco-current or uniflow direction, i.e. the direction of gas flow was thesame as the direction of catalyst travel, the quality of the resultingcatalyst was unsatisfactory. In that case the catalyst was cooled underthe gas atmosphere which had been produced by the activation (SO₂content several percent by volume) and the V₂ O₅ content was partlyreduced (the catalyst looked green). A catalyst of excellent quality wasproduced when the gas was sucked in a countercurrent direction. In thatcase the conditions of activation were comparable to those in thelaboratory experiments described in Examples 1 to 5: The activatedcatalyst was cooled in air with exclusion of the sulfur oxide producedduring the activating treatment.

The influence of temperature was investigated in 3 test series conductedin the range from 800° to 1100° C. The tests were conducted withconstant catalyst throughputs of 15, 30 and 60 kg/h.

The activity measurements had the following results:

    ______________________________________                                        Treating Conditions    Conversion                                             Temp.   Throughput rate   Activity                                            °C.                                                                            kg/h              %      at °C.                                ______________________________________                                        --      --                98.80  415                                          800     15                99.06  410                                          850     15                99.16  410                                          900     15                99.00  415                                          950     15                98.76  420                                          1000    15                97.80  440                                          800     30                98.95  415                                          850     30                99.05  410                                          900     30                99.15  410                                          950     30                99.02  415                                          1000    30                98.78  420                                          800     60                98.86  415                                          850     60                98.94  415                                          900     60                99.01  410                                          950     60                99.14  410                                          1000    60                99.12  410                                          1050    60                98.98  415                                          ______________________________________                                    

These results show that the optimum temperature to be used increasedwith an increase of the throughput rate in this Example from about 850°C. at a throughput rate of 15 kg/h via about 900° C. at a throughputrate of 30 kg/h to about 950°-1000° C. at a throughput rate of 60 kg/h.

It is believed that the fact that the conversion activity is higher byan absolute value of about 0.1% than in the case of the materialproduced on a laboratory scale (see Examples 1 and 4) is due to thefavorable flue gas atmosphere and the improved temperature profile inthe rotary kiln. That result has already been proved in Example 5.

EXAMPLE 7

Carrier material at a rate of 100 l/h was continuously impregnated in adrum which was heated to about 100° C. and was supplied with carrierbodies and impregnating solution at properly matched rates. Theimpregnating solution and the carrier material were the same as inExample 2.

The carrier material which had been moistened with the impregnatingsolution and at a temperature of about 100° C. was transferred over asuitable overflow device from the impregnating drum into a continuouslyoperating rotary kiln at that end which was remote from the burner. Therotary kiln was directly heated with a gas burner.

The flue gases were evacuated by means of a fan so that based on humidgas their O₂ content was less than 10 vol. %, 4 to 8 vol. % on theaverage, and the gases flowed countercurrently to the catalyst.

This operation resulted in a favorable heat transfer. The catalyst wasuniformly heated and the flue gases were correspondingly cooled at thesame time. When the catalyst had been dried, the aftertreatment wascarried out in the same equipment at the same time in that the feedingof the gas was so controlled that the highest catalyst temperature was850° to 900° C.

The catalyst was withdrawn on the burner side and was cooled with air ina succeeding cooling unit. With certain alterations, impregnating andcooling can be effected in the same rotary kiln. The catalyst wascomparable in quality to the catalyst produced in Example 6.

Determination of Conversion Activity

The testing equipment consists of jacketed tube, which is provided onthe outside with an electric heating coil. As the entering gas flowsthrough the space between the catalyst sample and the heating jacket,the gas is heated to an inlet temperature of about 500° C. The jacket isused to ensure that any local overheating of the catalyst will beprecluded. A displaceable thermocouple is disposed in the middle of thecatalyst sample. By a change of the heating coil and/or by a change ofthe heat insulation the temperatures are so adjusted that thetemperature gradient is as uniform as possible throughout the samplelayer. The temperature measured 5 cm above the sieve plate supportingthe sample is stated as the conversion temperature. The flowing gaspasses throughout the temperature range between 500° and 380° C. Theconversion is determined for decreasing temperatures in intervals ofabout 10 minutes and the resulting values are plotted on a graph onwhich the theoretical equilibrium conversion rate is also indicated.

The advantages afforded by the invention reside in that the inherentlygood conversion activity of a catalyst produced in two stages by animpregnation of prefabricated carrier bodies and intended for thereaction of SO₂ to SO₃ is considerably increased and variations inquality which occur during the production can be offset to a largeextent.

I claim:
 1. A process for producing a catalyst for convering SO₂ to SO₃which comprises the steps of:(a) impregnating silica prefabricatedcarrier bodies with an impregnating solution which contains 600 to 1100grams H₂ SO₄ and 140 to 830 grams V₂ O₅ and alkali metal sulfate perliter at a temperature between 60° C. and the boiling temperature of theimpregnating solution; (b) drying the impregnated carrier bodies withagitation at a temperature of up to 700° C. to form a V₂ O₅ and alkalimetal sulfate catalyst; and (c) activating said V₂ O₅ and alkali metalsulfate catalyst by heating it under oxidizing conditions at atemperature of 750° to 900° C. for 5 to 120 minutes in counterflow to anoxidizing gas atmosphere.
 2. A dried catalyst produced according to theprocess defined in claim
 1. 3. A process for producing a ctalyst forconverting SO₂ to SO₃ which comprises the steps of:(a) impregnatingsilica prefabricated carrier bodies with an impregnating solution whichcontains 600 to 1100 grams H₂ SO₄ and 140 to 830 grams V₂ O₅ and alkalimetal sulfate per liter at a temperature between 60° C. and the boilingtemperature of the impregnating solution; (b) drying the impregnatedcarrier bodies with agitation at a temperature of up to 700° C. to forma V₂ O₅ and alkali metal sulfate catalyst; and (c) activating said V₂ O₅and alkali metal sulfate catalyst by heating it under oxidizingconditions at a temperature of 750° to 1000° C. for 5 minutes incounterflow to an oxidizing gas atmosphere.
 4. A dried catalyst producedaccording to the process defined in claim 3.